An IPS-LCD is disclosed in U.S. Pat. No. 3,807,831. However, the IPS-LCD disclosed in U.S. Pat. No. 3,807,831 does not use a compensation film. Accordingly, the above IPS-LCD represents a low contrast ratio at a predetermined inclination angle due to a relatively great amount of light leakage in the black state of the IPS-LCD.
U.S. Pat. No. 5,189,538 discloses an LCD including a viewing angle compensation film using an A-plate and a positive biaxial retardation film.
In addition, U.S. Pat. No. 6,115,095 discloses an IPS-LCD including a compensation film using a +C-plate and an A-plate. Characteristics of the IPS-LCD disclosed in U.S. Pat. No. 6,115,095 are as follows:
1) A liquid crystal layer is horizontally aligned between two substrates in such a manner that an electric field is supplied in parallel to the liquid crystal layer by means of electrodes.
2) At least one A-plate and C-plate is sandwiched between two polarizer plate.
3) A main optical axis of an A-plate is perpendicular to a main optical axis of a liquid crystal layer.
4) A retardation value of a liquid crystal layer RLC, a retardation value of a + C-plate R+C, and a retardation value of an A-plate R+A are determined such that they satisfy the following equation.RLC:R+C:R+A≈1:0.5:0.25
5) A relationship between retardation values of the A-plate and C-plate and a thickness retardation value of a polarizer plate protective film (TAC, COP and PNB) is not disclosed.
A main object of using the A-plate and C-plate in U.S. Pat. No. 6,115,095 is to compensate for a color shift of the IPS-LCD in a white state (a bright state). In this case, although the color shift of the IPS-LCD in the white state can be reduced, a great amount of light leakage may occur at a predetermined inclination angle of the IPS-LCD in a black state. For this reason, the IPS-LCD represents a relatively low contrast ratio at the predetermined inclination angle.
Recently, various methods of reducing the color shift in the white state has been proposed and used. For example, a two-domain liquid crystal alignment using a zig-zag type electrode structure has been suggested in order to minimize the color shift in the white state.